PAX3 and PAX7 are closely related members of the paired box family of transcription factors. The gene products are expressed with overlapping yet distinct patterns during formation of the skeletal muscle lineage and have important roles in regulating growth, differentiation, and motility. Studies of the pediatric cancer alveolar rhabdomyosarcoma (ARMS) have demonstrated that PAX3 and PAX7 play important roles in neoplastic develpment involving the skeletal muscle lineage. In ARMS, the PAX 3 or PAX7 gene is fused to the FKHR gene by the t(2;13) or t(1;13) chromosomal translocation to created potent transcriptional activators with postulated oncogenic activity. In addition to these functional alterations, preliminary experiments have revealed distinct mechanisms for altering PAX3 and PAX7 expression in ARMS; the t(1;13) translocation is associated with amplification of the rearranged PAX7 gene, whereas the t(2;13) translocation is associated with overexpression of the rearranged PAX3 gene without amplification. Studies of embryonal rhabdomyosarcoma (ERMS), a second pediatric cancer related to the myogenic lineage, have revealed differences in wild-type PAX7 expression between ERMS and ARMS, and thus suggest tumor-specific differences in the respective transcriptional environments. To define the patterns of PAX3 and PAX7 expression in ARMS and ERMS, and elucidate the differences or alterations in expression, RNA and protein expression will be assaysed in tumor lines and specimens. As a starting-point for elucidating gene-and tumor-specific expression features, experiments will be conducted to determine the location of chromatin features on long-range genomic maps of the PAX3 and PAX7 loci. These resources will then be utilized in reporter-transfection assays to identify cis-acting elements that regulate PAX3 and PAX7 expression in these tumors. For those cis-acting elements which are responsible for altered expression of the rearranged genes or differential expression between ERMS and ARMS, the corresponding trans-acting factors will be studies by DNA binding and transcriptional assays. These studies will therefore combine mRNA detection, genomic mapping, and transcriptional approaches to define the pattern and molecular basis for control of PAX3 and PAX7 expression in these tumors, provide models for understanding their expression during normal skeletal muscle development, and ultimately enable the design of strategies to manipulate expression of these genes.